Virulence of many pathogens is affected by environment. In vitro culture often leads to loss of virulence, while animal passage permits expression of virulence factors. The in vivo environment is more complex than what is usually found in vitro, especially for intestinal organisms. One factor in the intestines, the microbial flora, is already known to limit the activity of potential pathogens, which usually pose little threat to a healthy individual but may cause antibiotic-associated diarrheas when the normal flora is disrupted. Long term antibiotic treatment is one way to disrupt the flora, then antibiotic-resistant microbes replace the normal flora and several pathogens begin to express activity. The proposed study will examine the interaction in vitro of antibiotic-resistant enteric bacteria with one recently recognized agent of antibiotic-associated gastroenteritis, Clostridium perfringens. In this study C. perfringens will be co-cultured with species of antibiotic resistant enterococci or enterobacilli and evaluated for sporulation, enterotoxin release, and vegetative growth. The experimental conditions will include both batch cultures and a continuous flow culture with antibiotic-resistant microbes or their secreted products. C. perfringens sporulation, enterotoxin production, and growth will be examined. The data should illustrate how microbial populations may affect the virulence of a second population, C. perfringens.